SPECIES. In logic the term “species” is applied to any group of individuals agreeing in some common attribute or attributes, and included along with other groups in a higher category, that of “genus,” which comprehends the fewer and more general attributes in which all agree and ignores those in which they differ. The application of these terms in logic is thus purely relative; any genus, however large, may be but a species of a still larger genus. But in arranging the innumerable objects of the natural sciences the naturalist finds it necessary to restrict the terms “species” and “genus” to the two lowest groupings and to distinguish the higher aggregates by special terms, as “family,” “order,” “class,” &c. Early writers had but a loose conception of many different “kinds” of animals and plants, and spoke only of species and genus in their purely logical relations, with varying breadth of content. The term “species” was limited to its natural history usage in the end of the 17th century by John Ray. His conception of “specific characters” rested, not only on close and constant resemblance in outward form, but also on the likeness of offspring to parent, a considerable measure of variability being, however, recognized. Amongst subsequent authors this conception of common descent or parentage became more and more prominent, while the progress of successful definition of species made the limits of their variability seem always narrower and of less importance; and in this way the useful working conception of the tolerable definiteness of species gradually crystallized into the absolute dogma of their fixity. Then Linnaeus in his Philosophia Botanica gave the aphorism “species tot sunt diversae, quot diversae formae ab initio sunt ...creatae” (we reckon just as many species as there were forms created at the beginning), which was generally accepted. Buffon’s obstinate rejection of the Linnaean classification was associated with a belief in the modifiability of species, and showed some foresight of the doctrine elaborated soon afterwards by Lamarck (q.v). The general acceptance of this dogma was, however, effected by the influence of Cuvier; its overthrow dates only from the publication of Darwin’s Origin of Species (1859), of which the argument need not be here repeated. (See Evolution, Morphology.) The genealogical conception of species was thus established more firmly than ever, though cleared from its former associations; in Haeckel’s phrase, the species is the whole succession of organisms which exhibit the same form in the same environment. The rash generalization, that distinct species are to be recognized by their incapacity for the production of fertile hybrids, was next overthrown, while closer study has cleared away the notion of the equal definiteness of all specific forms. We now know that, while many forms, like the pearly nautilus or the Venus’s fly-trap, do indeed exhibit the most perfect specific definiteness, the demarcation of equally definite species in other genera is rendered impossible by the existence of the most complete series of transitional forms, and the number of the species defined thus comes to depend simply on the personal equation of the systematist, on his predilection for “lumping” or “splitting,” as the case may be. Thus, for example, the number of described German species of hawkweed (Hieracium) has ranged from 300 for one author, through 106 for another and 52 for a third, to less than 20 for a fourth. Similar instances of variable genera are afforded by the willows and the brambles, and many other common forms. This wide variability, as might be expected, seems to be more prevalent among the lowest forms of life, and the classical example of the relativity and variability of species has been furnished by Haeckel’s beautiful monograph on the calcareous sponges (Monographie der Kalkschwämme, Jena, 1872), in which he offers twelve distinct arrangements of the same set of forms from various points of view, among which the two most nearly conventional propose respectively 21 genera and 111 species and 39 genera and 289 species. All such variable forms are in fact species in the making, which become definite in proportion as certain varieties become especially adapted to their environment, and become isolated by the dying out of the intermediate forms. With these limitations, however, the working usefulness of the morphological conception of species remains undiminished. The want of any absolute standard of specific difference is largely made up by practical experience and common sense; and the evolutionary systematists are less in danger than were their predecessors of either exaggerating or understating the importance of mere varieties. (See Variation.)
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